1. Field of the Invention
The present invention relates to a liquid crystal film and a liquid crystal display device equipped with such a liquid crystal film.
2. Description of the Prior Art
In recent years, as a result of the research development which has been actively conducted to apply liquid crystal materials for optical purposes, a liquid crystal film produced by aligning a liquid crystal material and forming the material into a film has been practically used for the purposes of compensating color and widening a viewing angle of a liquid crystal display device.
Various methods of forming a liquid crystal material into a film are known. For example, there is a method in which a liquid crystal film is produced by forming a thin film of a polymeric liquid crystalline substance over a substrate with alignability and then heating the film to a temperature equal to or higher than the glass transition temperature “Tg” so as to align the liquid crystal (liquid crystal molecules), followed by quenching the film so as to fix the aligned liquid crystal, as disclosed in Japanese Laid-Open Patent Publication No. 3-9321. However, although this method is applicable to both main chain- and side chain-type polymeric liquid crystalline substances, it has problems that the process would be increased in load and eligible alignment substrates are limited because the temperature at which the liquid crystallinity is exhibited rises when using a main chain-type polymeric liquid crystalline substance with a high Tg. Furthermore, when using a side chain-type polymeric liquid crystalline substance, this method has a problem in the heat resistance of the resulting liquid crystal film and a disadvantage that the orientation of the liquid crystal would be disordered at a temperature in the vicinity of the Tg.
Alternatively, there is a method in which a liquid crystal film is produced by sealing a liquid crystalline low molecular weight substance having a reactive group into a liquid crystal cell provided with an alignment substrate and heating the cell to a temperature at which the substance exhibits a liquid crystallinity so as to be aligned, followed by fixing the aligned liquid crystal by polymerization with an external stimulus such as light or heat, as disclosed in Japanese Laid-Open Patent Publication No. 8-21915. However, in this method, it is difficult to control the parameters of the resulting liquid crystal film because the liquid crystal to be aligned is a low molecular weight substance and thus the physical properties such as fluidity and birefringence are heavily dependent on temperature. Further, this method may often have a problem in the heat resistance of the resulting liquid crystal film because the liquid crystal after polymerization becomes similar in structure to a side chain-type polymeric liquid crystalline substance and thus fails to raise the Tg sufficiently.
As an alternative for fixing a liquid crystal orientation by polymerization with an external stimulus, there is a method in which a mixture of a polymeric liquid crystalline substance and a reactive low molecular weight substance is aligned and reacted, i.e., cured with an external stimulus, as disclosed in Japanese Laid-Open Patent Publication No. 10-120640. However, because the reaction between the polymeric liquid crystalline substance and the reactive low molecular weight substance does not progress sufficiently, the Tg is not sufficiently raised, leading to a problem in the heat resistance of the resulting liquid crystal film.
In view of these problems, it can be said that a method disclosed in Japanese Laid-Open Patent Publication No. 2000-319527 is the most effective in which method a reactive group is directly introduced into a polymeric liquid crystalline substance with a relatively low Tg, such as a side chain-type one and then cross-linked with an external stimulus such as light or heat after aligning the reactive group in a liquid crystal state, thereby raising the Tg. However, this method has a problem that it is difficult to synthesize such a side chain-type polymeric liquid crystalline substance with a reactive group. For example, when the polymeric structure is constructed prior to the introduction of a reactive group, the amount thereof is prone to be insufficient. On the other hand, in a method in which a side chain-type polymeric liquid crystalline substance having a reactive group is synthesized from a monomer having two reactive groups by polymerizing one of the reactive groups, it is necessary that the reactivity of the other reactive group must be maintained lower than that of the reactive group to be reacted, leading to a problem that the reaction of the reactive group after aligning the reaction group in a liquid crystal state is prone to be insufficient.
Furthermore, in the case of using only a side chain-type polymeric liquid crystalline substance, it may be difficult to shift the reactive region during a cross-linking reaction and thus there arises a problem that the cross-linking does not progress sufficiently.
The present invention provides a liquid crystal film with an excellent heat resistance, which can solve the above-described problems and a liquid crystal display device mounted with such a liquid crystal film.
The present invention was made in view of the forgoing problems and provides a liquid crystal film with an excellent heat resistance, and which is produced by fixing an aligned liquid crystal material containing a side chain-type polymeric liquid crystalline substance obtained by polymerizing a novel compound having reactive groups with excellent reactivity for fixing the aligned liquid crystal structure and a difunctional low molecular weight liquid crystalline substance having two or more such reactive groups.